Method for the Electrochemical Coating of a Workpiece

ABSTRACT

The invention relates to a method for the electrochemical coating of a workpiece through precipitation of an aluminum-containing metal layer from an ionic liquid, which contains aluminum ions, on the surface of the workpiece. In order to suggest measures, which enable an improvement of the surface properties of a workpiece coated with aluminum, it is provided that the ionic liquid contains particles and these particles are incorporated into the metal layer. The particles have a Mohs hardness of at least 5 and which are selected from silicic acid, aluminum oxide, titanium oxide particles, in particular of the rutile or anatase type, silicon oxide, zirconium oxide, tungsten carbide, chromium carbide, boron carbide, silicon nitride, silicon carbide and diamond particles as well as hollow micro-glass balls or a mixture of these and/or particles that contain lubricants and/or particles that contain graphene and/or fullerenes.

The invention relates to a method for the electrochemical coating of a workpiece.

Electrochemical methods represent one of the most important options of providing workpieces with metallic coatings. The workpiece is hereby inserted into a coating liquid, which contains metal ions. The liquid can be a melted mass of a salt of the corresponding metal; however, it frequently consists of a suitable solvent, in which ions are dissolved.

The coating liquid is contained in a coating reservoir during the coating. Furthermore, an electrode for the contacting of the workpiece as well as a counter electrode are immersed in the coating liquid within the reservoir. For the coating, voltage is applied between the workpiece and the counter electrode. The metal ions in the coating liquid are electrolytically discharged on the surface of the workpiece, whereby a metal layer is precipitated on the surface. The workpiece hereby forms a cathode, on which the metal cations are reduced.

A coating with aluminum is especially advantageous for numerous applications. This is due to the fact that, on one hand, an aluminum surface has an attractive appearance and, on the other hand, aluminum forms stable oxide layers, so that an aluminum coating can protect lower-lying metal from oxidation and corrosion.

Due to its low normal potential of −1.66 V, the precipitation of aluminum from an aqueous solution is not possible, since the hydrogen contained in the water is reduced instead. The search for alternative solvents that enable a precipitation of aluminum led to the so-called ionic liquids. These are salts, the melting point of which lies below 100° C., partially even below room temperature. Typical ionic liquids, which come into question for electrochemical coating processes with aluminum, are combinations of certain organic cations with halogenide ions, for example 1-ethyl-3-methylimidazolium chloride. In such a liquid, for example, aluminum chloride can be dissolved at room temperature. The solution can be used for the electrochemical precipitation of aluminum.

With the shown method, it is possible to provide metallic workpieces, such as steel screws, with an aluminum coating. However, the surface properties of the workpiece are hereby determined by those of the aluminum layer, which is disadvantageous for certain applications. Thus, aluminum with a Mohs hardness of 2.75 is one of the softest metals and thus susceptible to mechanical damage. Even the thin aluminum oxide layer forming in air does not offer sufficient protection in the case of heavier loads. Additionally, it is often desirable e.g. for parts like screws, nuts, hinges, etc. to guarantee a defined friction factor, which can be set as needed. This is not possible with an aluminum coating.

It is possible to apply in turn to the aluminum layer a coating based on an organic or inorganic binder, either as a powder or liquid paint or lacquer. Such a coating can in turn protect the aluminum layer. Moreover, the tribological properties of the surface can be set through lubricants like polytetrafluoroethylene, which are added to the paint.

It is hereby disadvantageous for one that the coating process becomes more complicated and expensive due to the application of the paint. The additional layer also contradicts the general effort to generate the thinnest possible coatings. Also, the adhesion of the paint layer on the aluminum layer is sometimes not as good as that of the aluminum layer on the substrate, which is in turn detrimental to the long-term durability of the coating.

DE 10 2008 031 003 discloses the generation of a CNT-rich coating on workpieces. WO 2009/016189 is concerned with the generation of particularly thick layers, which are precipitated from ionic liquids on workpiece surfaces. The strength of the layer should provide protection from stress.

The object of the invention is thus to suggest measures that enable the improvement of the surface properties of a workpiece coated with aluminum.

The object is solved through a method according to claim 1, a coating for a workpiece according to claim 15, a workpiece according to claim 16, an ionic liquid according to claim 17 and a method according to claim 18 or 19.

In the case of the method for the electrochemical coating of a workpiece according to the invention, an aluminum-containing layer is precipitated from an ionic liquid, which contains aluminum ions, on the surface of the workpiece. An example of such an ionic liquid is the aforementioned solution of aluminum chloride in 1-ethyl-3-methylimidazolium chloride (EMIMCl). However, apart from this, all ionic liquids can be used that are known from the state of the art for the precipitation of aluminum.

Available or respectively suitable ionic liquids include among others salts of 3-methylimidazolium with one of the following side chains

-   1-methyl (MMIM); 1-ethyl (EMIM), 1-propyl (PMIM), 1-butyl (BMIM),     1-hexyl (HMIM), 1-octyl (OMIM), 1-benzyl (ZMIM), 1-cyanomethyl     (MCNMIM), 1-(2-hydroxyethyl) (EOHMIM),     in particular with the following anions: -   [(C₂F₅)₃PF₃]⁻, abbreviation FAP (e.g. as HMIM FAP, EOHMIM FAP);     [N(SO₂CF₃)₂]⁻, abbreviation NTF (e.g. as EMIM NTF, BMIM NTF, HMIM     NTF, EOHMIM NTF, MCNMIM NTF); [CF₃SO₃]⁻, abbreviation OTF (e.g. as     EMIM OTF, BMIM OTF); [B(CN)₄]⁻, abbreviation TCB (e.g. as EMIM TCB);     [N(CN)₂]⁻, abbreviation DCN (e.g. as EMIM DCN, BMIM DCN); [C(CN)₃]⁻,     abbreviation TCC (e.g. as BMIM TCC); [SCN]⁻, abbreviation SCN (e.g.     as EMIM SCN); [HSO₄]⁻, abbreviation HSO4 (e.g. as EMIM HSO4, BMIM     HSO4); [CH₃SO₄]⁻, abbreviation MSU (e.g. as MMIM MSU, EMIM MSU, BMIM     MSU); [C₂H₅SO₄]⁻, abbreviation ESU (e.g. as EMIM ESU); [C₄H₉SO₄]⁻,     abbreviation BSU (e.g. as EMIM BSU); [C₆H₁₃SO₄]⁻, abbreviation HSU     (e.g. as EMIM HSU); [C₈H₁₇SO₄]⁻, abbreviation OSU (e.g. as EMIM OSU,     BMIM OSU); [C₅H₁₁O₂SO₄]⁻, abbreviation MEESU (e.g. as EMIM MEESU);     [B(C₂O₄)₂]⁻, abbreviation BOB (e.g. as EMIM BOB); [(CH₃)₂PO₄]⁻,     abbreviation DMP (e.g. as MMIM DMP); [(C₂H₅)₂PO₄]⁻, abbreviation DEP     (e.g. as EMIM DEP); [CH₃SO₃]⁻, abbreviation MSO (e.g. as EMIM MSO,     BMIM MSO); [CF₃COO]⁻, abbreviation ATF (e.g. as EMIM ATF, BMIM ATF);     [CH₃C₆H₄SO₃], abbreviation TOS (e.g. as EMIM TOS); [BF₄]⁻,     abbreviation BF4 (e.g. as EMIM BF4, BMIM BF4, HMIM BF4, OMIM BF4);     [PF₆]⁻, abbreviation PF6 (e.g. as BMIM PF6, HMIM PF6); Cl⁻,     abbreviation Cl (e.g. as EMIM Cl, BMIM Cl, HMIM Cl, OMIM Cl, ZMIM     Cl); Br⁻, abbreviation Br (e.g. as EMIM Br, BMIM Br, OMIM Br); I⁻,     abbreviation I (e.g. as PMIM I, BMIM I); [C₄F₉SO₃]⁻, abbreviation     NON.

Salts of 2, 3 dimethylimidazolium with one of the following side chains then also come into question:

-   1-methyl (MMMIM); 1-ethyl (EMMIM), 1-propyl (PMMIM), 1-butyl     (BMMIM), 1-hexyl (HMMIM),     in particular with the following anions: -   [(C₂F₅)₃PF₃]⁻, abbreviation FAP; [N(SO₂CF₃)₂]⁻, abbreviation NTF     (e.g. as PMMIM NTF); [CF₃SO₃]⁻, abbreviation OTF (e.g. as BMMIM     OTF); [B(CN)₄]⁻, abbreviation TCB; [N(CN)₂]⁻, abbreviation DCN;     [C(CN)₃]⁻, abbreviation TCC; [SCN]⁻, abbreviation SCN; [HSO₄]⁻,     abbreviation HSO4; [CH³SO₄]⁻, abbreviation MSU; [C₂H₅SO₄]⁻,     abbreviation ESU; [C₄H₉SO₄]⁻, abbreviation BSU; [C₆H₁₃SO₄]⁻,     abbreviation HSU; [C₈H₁₇SO₄]⁻, abbreviation OSU; [C₅H₁₁O₂SO₄]⁻,     abbreviation MEESU; [B(C₂O₄)₂]⁻, abbreviation BOB; [(CH₃)₂PO₄]⁻,     abbreviation DMP; [(C₂H₅)₂PO₄]⁻, abbreviation DEP; [CH₃SO₃]⁻,     abbreviation MSO; [CF₃COO]⁻, abbreviation ATF; [CH₃C₆H₄SO₃]⁻,     abbreviation TOS; [BF₄]⁻, abbreviation BF4 (e.g. as BMMIM BF4);     [PF₆]⁻, abbreviation PF6 (e.g. as BMMIM PF6); Cl⁻, abbreviation Cl     (e.g. as EMMIM Cl, BMMIM Cl, HMMIM Cl); Br⁻, abbreviation Br; I,     abbreviation I (e.g. as MMMIM I, BMMIM I); [C₄F₉SO₃]⁻, abbreviation     NON.

The salts of 1,3 dimethylimidazolium (MMIM) also come into consideration, in particular with the following anions:

-   [CH₃SO₄]⁻, abbreviation MSU, e.g. as (MMIM DMP); [(CH₃)₂PO₄]⁻,     abbreviation DMP, e.g. as (MMIM MSU).

The salts of 3-methyl octylimidazolium (OMIM) then come into consideration, in particular as chloride (OMIM Cl) or as tetrafluoroborate (OMIM BF4), but also the iodized salts of 3-methyl-1-propylimidazolium (PMIM I) or 1, 2, 3 trimethylimidazolium (MMIM I).

Salts of pyridinium with one of the following side chains then also come into question:

-   N-butyl (BPYR), N-ethyl-3-methyl (E3MPYR), N-butyl-3-methyl     (B3MPYR), N-butyl-4-methyl (B4MPYR), N-(3-hydroxypropyl) (POHPYR),     N-hexyl-4-(dimethylammino) (HDMAP), N-ethyl-3-hydroxymethyl (EHMP),     N-hexyl (HPYR),     in particular with the following anions: -   [(C₂F₅)₃PF₃]⁻, abbreviation FAP (e.g. as POHPYR FAP); [N(SO₂CF₃)₂]⁻,     abbreviation NTF (e.g. as POHPYR NTF, HPYR NTF, HDMAP NTF);     [CF₃SO₃]⁻, abbreviation OTF (e.g. as B3MPYR OTF); [B(CN)₄]⁻,     abbreviation TCB; [N(CN)₂]⁻, abbreviation DCN (e.g. as B3MPYR DCN);     [C(CN)₃]⁻, abbreviation TCC; [SCN]⁻, abbreviation SCN; [HSO₄]⁻,     abbreviation HSO4; [CH₃SO₄]⁻, abbreviation MSU (e.g. as B3MPYR MSU);     [C₂H₅SO₄]⁻, abbreviation ESU (e.g. as E3MPYR ESU, EHMP ESU);     [C₄H₉SO₄]⁻, abbreviation BSU; [C₆H₁₃SO₄]⁻, abbreviation HSU;     [C₈H₁₇SO₄]⁻, abbreviation OSU; [C₅H₁₁O₂SO₄]⁻, abbreviation MEESU;     [B(C₂O₄)₂]⁻, abbreviation BOB; [(CH₃)₂PO₄]⁻, abbreviation DMP;     [(C₂H₅)₂PO₄], abbreviation DEP; [CH₃SO₃], abbreviation MSO;     [CF₃COO]⁻, abbreviation ATF; [CH₃C₆H₄SO₃]⁻, abbreviation TOS;     [BF₄]⁻, abbreviation BF4 (e.g. as B3MPYR BF4, B4MPYR BF4); [PF₆]⁻,     abbreviation PF6 (e.g. as B3MPYR PF6); Cl⁻, abbreviation Cl (e.g. as     BPYR Cl, B3MPYR Cl); Br⁻, abbreviation Br (e.g. as B3MPYR Br); I,     abbreviation I; [C₄F₉SO₃]⁻, abbreviation NON (e.g. as B3MPYR NON,     E3MPYR NON).

Salts of N-methyl-pyrrolidinium with one of the following side chains then also come into question:

-   N-methyl (MMPL), N-butyl (BMPL), N-methyl-1-octyl (OMPL), N-hexyl     (HMPL), N-(6-aminohexyl) (HNH2MPL), N-(2-methoxyethyl) (MOEMPL),     in particular with the following anions: -   [(C₂F₅)₃PF₃]⁻, abbreviation FAP (e.g. as BMPL FAP, MOEMPL FAP);     [N(SO₂CF₃)₂]⁻, abbreviation NTF (e.g. as BMPL NTF, HMPL NTF, MOEMPL     NTF, HNH2MPL NTF); [CF₃SO₃]⁻, abbreviation OTF (e.g. as BMPL OTF);     [B(CN)₄]⁻, abbreviation TCB (e.g. as BMPL TCB); [N(CN)₂]⁻,     abbreviation DCN (e.g. as BMPL DCN); [C(CN)₃]⁻, abbreviation TCC;     [SCN]⁻, abbreviation SCN; [HSO₄]⁻, abbreviation HSO4; [CH₃SO₄],     abbreviation MSU (e.g. as B3MPYR MSU); [C₂H₅SO₄]⁻, abbreviation ESU     (e.g. as E3MPYR ESU, EHMP ESU); [C₄H₉SO₄]⁻, abbreviation BSU;     [C₆H₁₃SO₄]⁻, abbreviation HSU; [C₈H₁₇SO₄]⁻, abbreviation OSU;     [C₅H₁₁O₂SO₄]⁻, abbreviation MEESU; [B(C₂O₄)₂]⁻, abbreviation BOB     (e.g. as BMPL BOB); [(CH₃)₂PO₄]⁻, abbreviation DMP; [(C₂H₅)₂PO₄]⁻,     abbreviation DEP; [CH₃SO₃]⁻, abbreviation MSO; [CF₃COO]⁻,     abbreviation ATF (e.g. as BMPL ATF); [CH₃C₆H₄SO₃]⁻, abbreviation     TOS; [BF₄]⁻, abbreviation BF4; [PF₆]⁻, abbreviation PF6; Cl⁻,     abbreviation Cl (e.g. as BMPL Cl, OMPL Cl); Br⁻, abbreviation Br     (e.g. as BMPL Br); I⁻, abbreviation I (e.g. as MMPL I); [C₄F₉SO₃],     abbreviation NON.

Salts of ammonium N (R, R′, R″, R′″) with one of the following side chains then come into question:

-   tetramethyl (NM4), tetrabutyl (NB4), methyltrioctyl (N(03)M),     ethyl-dimethyl-propyl (NEMMP), N-ethyl-N,N-dimethyl-2-methoxyethyl     (MOEDEA), (2-hydroxyethyl)trimethyl (CHOLINE),     hydrazinocarbonyl-methyl-trimethyl (GIRT),     ethyl-dimethyl-(5-diisorpropylamino-3-oxapentyl) (DIAN),     ethyl-dimethyl-cyanomethyl (MCNDEA),     in particular with the following anions: -   [(C₂F₅)₃PF₃]⁻, abbreviation FAP (e.g. as NM4 FAP, MOEDEA FAP);     [N(SO₂CF₃)₂]⁻, abbreviation NTF (e.g. as NB4 NTF, N(03)M NTF, NEMMP     NTF, MOEDEA NTF, GIRT NTF, DIAN NTF, MCNDEA NTF); [CF₃SO₃],     abbreviation OTF; [B(CN)₄]⁻, abbreviation TCB; [N(CN)₂]⁻,     abbreviation DCN; [C(CN)₃]⁻, abbreviation TCC; [SCN]⁻, abbreviation     SCN; [HSO₄]⁻, abbreviation HSO4; [CH₃SO₄]⁻, abbreviation MSU;     [C₂H₅SO₄]⁻, abbreviation ESU; [C₄H₉SO₄]⁻, abbreviation BSU;     [C₆H₁₃SO₄]⁻, abbreviation HSU; [C₈H₁₇SO₄]⁻, abbreviation OSU;     [C₅H₁₁O₂SO₄]⁻, abbreviation MEESU; [B(C₂O₄)₂]⁻, abbreviation BOB;     [(CH₃)₂PO₄]⁻, abbreviation DMP (e.g. as CHOLINE DMP); [(C₂H₅)₂PO₄]⁻,     abbreviation DEP; [CH₃SO₃]⁻, abbreviation MSO; [CF₃COO]⁻,     abbreviation ATF (e.g. as N(03)M ATF); [CH₃C₆H₄SO₃]⁻, abbreviation     TOS; [BF₄], abbreviation BF4; [PF₆], abbreviation PF6; Cl⁻,     abbreviation Cl; Br⁻, abbreviation Br; I⁻, abbreviation I;     [C₄F₉SO₃]⁻, abbreviation NON.

Salts of guadininium (GUA), N-(methoxyethyl)-N-methyl-morpholinium (MOEMMO), 1-(methoxyethyl)-1-methyl-piperidinium (MOEMPIP), trihexyl(tetradecyl)phosphonium (P(h3)t) and triethyl-sulfonium (SE3) can also be used,

in particular with the following anions:

-   [(C₂F₅)₃PF₃]⁻, abbreviation FAP (e.g. as GUA FAP, MOEMMO FAP,     MOEMPIP FAP, P(h3)t FAP); [N(SO₂CF₃)₂]⁻, abbreviation NTF (e.g. as     MOEMMO NTF, MOEMPIP NTF, P(h3)t NTF, SE3 NTF); [CF₃SO₃]⁻,     abbreviation OTF; [B(CN)₄]⁻, abbreviation TCB; [N(CN)₂]⁻,     abbreviation DCN; [C(CN)₃]⁻, abbreviation TCC; [SCN]⁻, abbreviation     SCN; [HSO₄]⁻, abbreviation HSO4; [CH₃SO₄]⁻, abbreviation MSU;     [C₂H₅SO₄]⁻, abbreviation ESU; [C₄H₉SO₄]⁻, abbreviation BSU;     [C₆H₁₃SO₄], abbreviation HSU; [C₈H₁₇SO₄], abbreviation OSU;     [C₅H₁₁O₂SO₄]⁻, abbreviation MEESU; [B(C₂O₄)₂]⁻, abbreviation BOB;     [(CH₃)₂PO₄]⁻, abbreviation DMP (e.g. as DMP CHOLINE); [(C₂H₅)₂PO₄]⁻,     abbreviation DEP; [CH₃SO₃]⁻, abbreviation MSO; [CF₃COO]⁻,     abbreviation ATF (e.g. as ATF N(03)M); [CH₃C₆H₄SO₃]⁻, abbreviation     TOS; [BF₄]⁻, abbreviation BF4; [PF₆]⁻, abbreviation PF6; Cl⁻,     abbreviation Cl; Br⁻, abbreviation Br; I⁻, abbreviation I;     [C₄F₉SO₃]⁻, abbreviation NON; [(C₂F₅)₃PF₃]⁻, abbreviation FAP.

It can hereby be provided that an aluminum bond is dissolved, the anions of which correspond with that of the ionic liquid. However, this is not mandatory.

Workpieces with a metallic surface, which are protected from corrosion by the layer containing aluminum, primarily come into question as workpieces. The term metallic hereby includes all metals and alloys, in particular all types of steels. A person skilled in the art understands that an alloy according to the state of the art can, in addition to metals, also include semi-metals or non-metals, for example silicon or carbon. It is also conceivable that it is a workpiece that already has a metal layer, i.e. was e.g. previously electrochemically coated. Besides metallic surfaces, non-metallic surfaces also come into question, provided they are conductive. For example, conductive plastics as well as so-called organic semi-conductors, which have sufficient conductivity in order to permit electrochemical coating, are known from the state of the art.

According to the invention, the ionic liquid contains particles. According to the established opinion, the term particle here describes small solid bodies, the largest expansion of which is at most 1 mm. The particles can have various shapes, such as ball-shaped or ball-like, lamellar, lenticular, polyhedral or spicular. The particles can generally have any composition; however, materials that do not show decomposition in ionic liquids and are in particular no soluble in them are preferred. Preferred materials and size of particles are discussed below.

The ionic liquid used according to the invention can be prepared by adding particles to an ionic liquid containing aluminum ions. Alternatively, an ionic liquid can be prepared to which particles are added, whereupon aluminum ions are dissolved in the ionic liquid. In order to protect the ionic liquid from contamination, the particles should be added in as pure a form as possible (e.g. not as a paste or the like). In the case of ionic liquids that are sensitive to water, the particles should be cleaned by a drying of adhering air humidity. The addition of particles can take place on site right before the coating but it can also take place in the factory, whereby a prepared coating bath can be made available.

The particles are preferably suspended in the liquid, i.e. they do not sink but rather float in it. Within the precipitation process, the particles are incorporated into the metal layer containing the aluminum.

The exact mechanism of the incorporation can be different. Thus, if applicable, the precipitated metal can function as a type of “binder,” through which particles that happen to be located in the immediate vicinity of the surface of the workpiece are tied to them. It is also conceivable that particles also wander electrostatically through the electrical field prevailing during the precipitation to the surface of the workpiece, where they are tied by the precipitated metal. The object of the present invention is however independent of the mechanisms underlying the incorporation. In any case, the metal layer precipitated on the workpiece forms a type of matrix, into which the particles are integrated. The incorporation can hereby be incomplete, i.e. particles can e.g. project from the surface of the metal layer, which are thus only partially incorporated.

Through the incorporation of particles according to the invention, it is possible to make the surface properties of the metal layer flexible. More precisely, a surface property of the metal layer is set through the incorporation of the particles. It is thus possible to have planned and targeted impact on the surface property. On one hand, this is based on the fact that one part of the particles normally projects out of the surface of the metal layer. On the other hand, particles fully incorporated in the layer can have an impact on the surface properties, in particular when the incorporation takes place close to the surface. Even particles that at first lie tightly below the surface of the metal layer can be released through abrasion during the use of the workpiece and thus impact the properties of the surface afterwards. Preferred options of the surface design will be explained in greater detail below. The incorporation during the precipitation process ensures optimal bonding of the particles to the metal layer and thus to the surface of the workpiece.

Another advantage is the possibility that the surface design is possible through one single layer. An additional coating step is hereby omitted and it is generally possible to design the coating thinner than in conventional processes in which at least one additional layer must be applied. This saves time, money and material. It should also be noted that potential problems with the adhesion of an additional layer on the metal are avoided.

The process is particularly suitable for the incorporation of the hard material particles named below. A higher effective hardness of the coating is hereby set through the incorporated particles. The aluminum layer, which forms the matrix for the particles, is effectively protected from abrasion. This results from the fact that—either from the very beginning or after wear of the above-lying aluminum—hard material particles project from the surface of the aluminum layer. Exterior abrasive impacts primarily impact the particles, whereby the aluminum layer and thus also the lower-lying substrate are protected. According to the invention, the wear resistance and/or the abrasion resistance of the coating can be increased much more in this manner even for thin layers than through an increase in the layer thickness.

Such particles typically have a Mohs hardness of at least 5, preferably at least 7, particularly preferably at least 9. The particles are advantageously selected from silicic acid, aluminum oxide, titanium oxide, silicon oxide, zirconium oxide, tungsten carbide, chromium carbide, boron carbide, silicon nitride, silicon carbide and diamond particles as well as hollow micro-glass balls or a mixture of these.

Titanium oxide particles of the rutile or anatase type are advantageous. They form radicals through photocatalysis on light, e.g. OH radicals from water, whereby organic substances are broken down. A workpiece coated in this manner thus has a self-cleaning ability, which contributes to the protection of the surface.

It is also particularly advantageous when particles including lubricants are used. Such particles can consist fully of lubricant or also partially, i.e. e.g. comprise an aluminum oxide core, which is coated with lubricant. It is understood that the term lubricant here describes such substances that are solid at temperatures that are typical for the process, i.e. e.g. between 0° C. and 100° C. The use of lubricant particles can for one be used to set a specified friction coefficient for workpieces such as screws. Lubricants can also be used to prevent mechanical damage to the workpiece or increase wear, since friction forces that are often the cause of damage can hereby be lowered. The use of lubricants here shows a greater impact than an increase in the layer thickness of the surface coating.

All substances known from the state of the art come into question as lubricants, thus e.g. halogen carbon hydrides, in particular polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), perfluoroalkoxy copolymer (PFA), copolymer of tetrafluoroethylene with perfluorinated propylene and perfluoroalkylvinylether (EPE), copolymer of tetrafluoroethylene and perfluoromethylvinylether (MFA), MoS₂, boron nitride, graphite, fluorinated graphite, carnauba wax, polysulfones, polyolefin resins, in particular polyethylene (PE) and polypropylene (PP), mixtures of the same or a combination of these.

In the case of the present method, particles that comprise graphene and/or fullerenes are preferably used. The named carbon modifications can have a positive impact on the surface properties of the metal layer in a different manner. Thus, e.g. due to their high mechanical stability, they can protect the metal layer from wear.

It is also expressly provided to combine different particle types. Thus, a workpiece can be protected particularly effectively from abrasion e.g. through the combined incorporation of hard material particles and lubricant particles.

The size of the particles used can vary relatively widely. However, it should be noted that particles that are too large cannot be well integrated into the metal layers that are typically just a few micrometers thick, while particles that are too small can be ineffective for the setting of the surface properties. It is thus advantageous when particles are used that have a maximum expansion between 10 nm and 10 μm, preferably between 50 nm and 5 μm, more preferably between 100 nm and 2 μm, especially preferably between 500 nm and 1 μm.

The thickness of the coating is generally 1 μm to 5 μm, preferably 1 μm to 3 μm.

If nano particles are used, i.e. particles considerably below 1 μm, they are also preferably used in ionically stabilized form.

The percent by weight of the particles is advantageous between 0.1% and 10%, preferably between 1% and 8%, more preferably between 2% and 5%, with respect to a reactive component contained in the ionic liquid. Reactive component hereby describes the part of the ionic liquid that participates in the precipitation. Thus, if there e.g. is a melted mass of EMIMCl, in which the aluminum chloride is dissolved, then the aluminum chloride is the reactive component. Lower or higher particle percentages within the ionic liquid can have a disadvantageous impact on the percentage of particles within the precipitated metal layer. In order to not weaken the structural cohesion of the metal layer, the percentage of particles in the layer should not be too large. On the other hand, if the percentage is too low, the particles may no longer have sufficient impact on the surface properties.

The ionic liquids used in the present method are often subject to a sedimentation process, which leads to the formation of an inhomogeneous concentration of the metal ions to be precipitated. This can disadvantageously impact the precipitation results. But, in the case of the method according to the invention, it can in particular also happen that the particles do not remain suspended homogeneously in the ionic liquid, but rather sink to the bottom or float to the top depending on whether their density is greater or less than that of the ionic liquid. Both effects are generally opposed to a controlled incorporation process. In order to counteract these effects, it is preferred that the ionic liquid is thoroughly mixed before, during and/or after the precipitation. The ionic liquid and in particular the distribution of the particles within it can hereby be effectively homogenized.

A mixing before and/or after the precipitation offers the advantage that the ion flows induced by the electrical field, which support the precipitation process, are not superimposed by a movement of the liquid. The thorough mixing during the precipitation process can also lead to the poorer adsorption of particles on the surface, since they are ripped away before a bonding by the precipitated metal can take place.

On the other hand, it can be advantageous to perform a thorough mixing also during precipitation, in particular when the precipitation process takes a long time, the particles are relatively large and their density considerably exceeds that of the ionic liquid, in order to keep the particle concentration somewhat homogeneous. Of course, it is possible to interrupt the precipitation process for this.

Mixing can be performed for example by mechanical stirrers, magnetic stirrers or through sound, in particular ultrasound. It is also conceivable that the entire coating reservoir, in which the ionic liquid is located, is turned (e.g. like a concrete mixing machine) or is oscillated back and forth, in order to achieve a thorough mixing of the ionic liquid. Also the workpiece located in the ionic liquid can itself be moved for this purpose.

In a preferred embodiment of the method, a percent by weight, a percent by volume and/or a number of particles are monitored in the ionic liquid. Since many ionic liquids are transparent, such monitoring can e.g. be performed with a nephelometer. The fact that the dispersion of light shone into the ionic liquid is changed by the number of particles suspended in it is hereby taken advantage of. Alternatively or additionally, sample particle counts can also be performed in a small partial volume of the liquid. It is hereby also possible to identify, e.g. spectroscopically, the composition of individual particles in order to perform a separate monitoring for the individual types in the case of the use of different particles. Other methods for measuring the number of particles are also suitable, which are known in the state of the art.

In the simplest case, the monitoring can take place manually. However, automatic monitoring is preferred. The monitoring can hereby also be used for the automatic setting of the percent by volume or the number of particles, as will be explained below. A parallel monitoring for different areas of the ionic liquid is also conceivable. It is hereby possible to determine whether the distribution of particles is too inhomogeneous and thus thorough mixing is necessary.

In order to allow for the fact that the ionic liquid loses particles through incorporation into the metal layer, it is provided in a further embodiment of the method that in the case of a continuously used immersion bath particles are added to set the number, the percent by weight and/or percent by volume of the particles of the ionic liquid. This can take place before, during and/or after the precipitation. Since it is difficult to add the particles such that they are distributed evenly in the ionic liquid from the very beginning, it is hereby preferred to also perform a thorough mixing of the ionic liquid. The addition of the particles can take place based on calculations or empirical values. However, it is performed particularly advantageously dynamically depending on the results of the monitoring of the percent by volume or the number of particles. In this case, monitoring and supplementation of the particles form parts of a control circuit.

In a further development of the method, the ionic liquid comprises ions at least of another element, which is selected from the group consisting of silicon, iron, copper, manganese, magnesium, chromium, nickel, zinc, lead and titanium. During the precipitation process, an aluminum alloy rather than pure aluminum is precipitated in this case so that the metal layer contains at least one of the named elements in addition to aluminum. The aluminum alloy preferably contains at least 60 wt.-%, more preferably at least 70 wt.-%, particularly preferably at least 80 wt.-%, advantageously at least 90 wt.-% aluminum.

As known from the state of the art, it is often advantageous in the case of the present invention to pretreat the surface of the workpiece before applying the metal layer. Degreasing, sand-blasting, shot-blasting, phosphating or in-situ electrochemical etching (e.g. electrochemical polishing) of the surface is hereby conceivable.

After applying the metal layer, other process steps are also possible, through which the quality of the coating can be improved even further or modified for certain requirements. For one, the aluminum-containing metal layer can be post-treated, in particular through painting or dyeing, sealing or application of a top coat.

With the method according to the invention, a coating for a workpiece can be generated, which comprises an aluminum-containing metal layer on the surface of the workpiece generated through electrochemical precipitation. According to the invention, particles can hereby be incorporated into the metal layer.

Details of the invention will be explained below based on an exemplary embodiment with reference to the figure.

FIG. 1 shows a schematic representation of an embodiment of a coating apparatus for performing the method according to the invention.

EXAMPLE 1

Steel screws 100 are provided with aluminum for coating, wherein the surface of the aluminum layer is to be protected from wear through incorporation of silicon carbide particles 11.

The screws 100 are sand-blasted and degreased in a hot alkaline manner. Then comes a rinsing procedure with tap water, followed by a subsequent drying with hot air. The screws 100 are transferred into an air-tight, closable coating chamber, which has a nitrogen atmosphere with a relative residual moisture below 0.1%.

The coating apparatus 1 shown in FIG. 1 is located in the coating chamber. It comprises a reservoir 2 made of ceramic material, which is insensitive to ionic liquids. In its operating state, the reservoir 2 is filled with a coating liquid 10, which is made up of a mixture of EMIMCl and aluminum chloride at a ratio of 1.5:1.

A receiving device 3 for the screws 100 is arranged in reservoir 2. The receiving device 3 is cylindrical and cup-like with a penetrated wall. It can be moved vertically and can also be turned on its symmetrical axis, the position of which can also be set vertically. On the inside, the receiving device 3 has a plurality of electrodes (not shown) for contacting the screws 100. These electrodes are connected by means of a voltage source (not shown) with a counter electrode 4 made of ultra-pure aluminum, which is washed around by the coating liquid 10.

A plurality of silicon carbide particles 11 is suspended in the coating liquid 10. The approximately spherical particles 11 have an average diameter of approx. 500 nm. In order to prevent the settling of the particles 11, the coating liquid 10 is continuously mixed by means of a magnetic stirrer 5. The percent by weight of the particles 11 with respect to the reactive component contained in the coating liquid 10 is 2%.

FIG. 1 shows the state of the apparatus during the coating process. The screws 100 are located in the receiving device 3, which was moved into the coating liquid 10 and the axis of which is shifted vertically approx. 45°. During the coating process, the receiving device 3 is rotated with approx. 5 rpm around its symmetrical axis. This results in a circulation of the screws 100, whereby permanent contact points between the screws 100 are avoided.

For the coating, a voltage, which generates a current density of 20 mA/cm², is applied by means of the voltage source between the electrodes in the wall and the aluminum electrode 4, wherein the aluminum electrode 4 functions as an anode. This results in a precipitation of aluminum on the surface of the screws 100, while aluminum ions dissolve continuously from the counter electrode 4, so that the aluminum concentration in the coating liquid 10 remains constant.

Since a plurality of particles 11 is suspended in the coating liquid 10, they are also always located near the surface of the screws 100, there where the precipitation occurs. The forming aluminum layer thus grows around the particles 11 that are close to the surface and surrounds them. An aluminum layer thus forms on the screws 100, into which silicon carbide particles 11 are incorporated.

Since average dimensions and the density of the particles 11 are known, a certain number of particles corresponds with a specified percent by weight. This is monitored in that a test volume 6 of a few cubic millimeters is examined in a recess of the reservoir wall. The magnifying optics of a special camera 7 is pointed at the test volume. Since the coating liquid 10 is transparent, the particles 11 can be captured by the camera 7. An image recognition software connected to the camera 7 hereby performs regular particle counts. If it is hereby determined that the number of particles permanently exceeds a specified setpoint value, additional particles 11 are taken from a reservoir 9 and added to the coating liquid 10 via a dosing device 8.

After an aluminum layer of a sufficient thickness has been precipitated, the voltage source is switched off and the rotation of the receiving device 3 is stopped. The receiving device 3 is moved vertically out of the coating liquid 10 and is tipped into a basket (not shown) by changing its inclination.

The screws 100 now have an anticorrosive aluminum layer, which is protected from wear, since a plurality of silicon carbide particles 11 project from the surface of the aluminum layer and thus ensure an increased effective hardness.

The screws 100 can, as already shown, be then post-treated in different manners. It is also possible to perform an electrochemical polishing of the screws 100 before the coating process in that the screws 100 are switched as anodes.

EXAMPLE 2

A batch of steel screws 100 is to be coated with aluminum. Through the introduction of PTFE, the surface of the steel screws is set to an even friction factor. 1% PTFE particles with a diameter of 500 nm are dispersed in the coating liquid.

The coating takes place as explained in example 1. The applied coating has a layer thickness of 3 μm. The surface has an even friction factor. 

1-18. (canceled)
 19. A method for the electrochemical coating of a workpiece through precipitation of an aluminum-containing metal layer from an ionic liquid, which contains aluminum ions, on the surface of the workpiece, wherein the ionic liquid contains particles and the particles are incorporated into the metal layer, and wherein a surface property of the metal layer is set through incorporation of the particles, wherein the particles: have a Mohs hardness of at least 5 and are selected from the group consisting of particles of silicic acid, aluminum oxide, titanium oxide particles, silicon oxide, zirconium oxide, tungsten carbide, chromium carbide, boron carbide, silicon nitride, silicon carbide and diamond, hollow micro-glass balls and combinations thereof.
 20. The method according to claim 19, wherein the particles have a Mohs hardness of at least
 7. 21. The method according to claim 19, wherein the particles contain lubricants.
 22. The method according to claim 19, wherein the particles contain lubricant selected from the group consisting of halogen carbon hydrides, MoS₂, boron nitride, graphite, fluorinated graphite, carnauba wax, polysulfones, polyolefin resins, and mixtures thereof.
 23. The method according to claim 19, wherein the particles have a maximum expansion between 10 nm and 10 μm.
 24. The method according to claim 19, wherein the layer thickness of the applied layer is 1 μm to 5 μm.
 25. The method according to claim 19, wherein an ionic liquid is used, in which the percent by weight of the particles is between 0.1% and 10%, with respect to a reactive component contained in the ionic liquid.
 26. The method according to claim 19, wherein a thorough mixing of the ionic liquid takes place before, during and/or after the precipitation.
 27. The method according to claim 19, wherein a percent by weight, a percent by volume and/or a number of the particles is monitored in the ionic fluid.
 28. The method according to claim 19, wherein particles are added before, during and/or after the precipitation for the setting of the number, the percent by weight and/or the percent by volume of the particles of the ionic liquid.
 29. The method according to claim 19, wherein an ionic liquid is used which contains ions of at least one additional element selected from the group consisting of silicon, iron, copper, manganese, magnesium, chromium, nickel, zinc, lead and titanium and wherein a metal layer is precipitated, which contains the at least one additional element.
 30. The method according to claim 19, wherein the surface of the workpiece is pretreated before the application of the metal layer through degreasing, sand-blasting, shot-blasting, in-situ electrochemical etching, phosphating or application of an adhesive agent.
 31. The method according to claim 19, wherein after precipitation of the metal layer, the layer is post-treated through painting or dyeing, sealing or application of a top coat.
 32. The method according to claim 19, wherein the particles contain graphene and/or fullerenes.
 33. A coating for a workpiece, comprising an aluminum-containing metal layer generated through electrochemical precipitation on the surface of the workpiece, wherein particles are incorporated into the metal layer, wherein the particles: have a Mohs hardness of at least 5 and are selected from the group consisting of particles of silicic acid, aluminum oxide, titanium oxide particles, silicon oxide, zirconium oxide, tungsten carbide, chromium carbide, boron carbide, silicon nitride, silicon carbide and diamond, hollow micro-glass balls or combinations thereof.
 34. A workpiece with a coating according to claim
 33. 35. An ionic liquid, containing aluminum ions, wherein the ionic liquid contains particles wherein the particles: have a Mohs hardness of at least 5 and are selected from the group consisting of particles of silicic acid, aluminum oxide, titanium oxide particles, silicon oxide, zirconium oxide, tungsten carbide, chromium carbide, boron carbide, silicon nitride, silicon carbide and diamond, hollow micro-glass balls and combinations thereof.
 36. A method for the provision of an ionic liquid according to claim 35, comprising the steps: preparation of an ionic liquid which contains aluminum ions addition of particles to the ionic liquid, wherein the particles comprise: particles, which have a Mohs hardness of at least 5 and which are selected from the group consisting of particles of silicic acid, aluminum oxide, titanium oxide particles, silicon oxide, zirconium oxide, tungsten carbide, chromium carbide, boron carbide, silicon nitride, silicon carbide and diamond, hollow micro-glass balls and combinations thereof.
 37. A method for the provision of an ionic liquid according to claim 35, comprising: preparation of an ionic liquid addition of particles to the ionic liquid, wherein the particles comprise: particles, which have a Mohs hardness of at least 5 and which are selected from the group consisting of particles of silicic acid, aluminum oxide, titanium oxide particles, silicon oxide, zirconium oxide, tungsten carbide, chromium carbide, boron carbide, silicon nitride, silicon carbide and diamond, hollow micro-glass balls and combinations thereof; and dissolving aluminum ions in the ionic liquid. 